Conventional ultrasound scanners create two-dimensional gray-scale (B-mode) images which represent a thin slice through an area of anatomy. Flow information can also be extracted by detecting the Doppler shift between transmitted and returned ultrasound pulses. A conventional ultrasound system displays flow as either average Doppler power (power Doppler imaging) or average velocity (commonly referred to as color flow imaging). A flow image is conventionally displayed as an overlay on a B-mode image.
The transmitted pulses in power Doppler and color flow imaging are typically more narrowband than B-mode pulses in order to gain Doppler sensitivity. Operating on a packet of as many as 16 transmits, a high-pass wall filter rejects echoes from slower-moving tissue or vessel walls to reduce the signal dynamic range. The number of wall filter output samples per packet is given by (N-W+1), where N is packet size and W is wall filter length. Subsequently, the instantaneous Doppler power is computed as the magnitude squared of each wall filter quadrature output and the average of all outputs yields the average Doppler power. Alternatively, the average velocity is computed from the wall filter quadrature output based on the Doppler principle or time delay. The Kasai autocorrelation algorithm or a cross-correlation algorithm can be used to estimate the average flow velocity.
Although conventional color-flow imaging has very good flow sensitivity, the ability to see physical flow is limited by its limited dynamic range (which is partially dependent on the compression curve), limited resolution (due to narrowband pulses), limited frame rate (due to large packet sizes), and axialonly flow sensitivity (which is dictated by the reliance on the Doppler effect). In addition, conventional color-flow imaging suffers from artifacts such as aliasing, color blooming and bleeding.
If the ultrasound probe is swept over an area of body and the slices are stored in memory, a three-dimensional data volume can be acquired containing both the B-mode and flow information. The data volume can be used to project a three-dimensional view of the area of interest. The quality of three-dimensional projections of color flow and power Doppler data (either separately or combined with B-mode data) suffer for the aforementioned reasons, namely, reduced acoustic frame rate, Doppler sensitivity, reduced flow resolution, and "flash" artifacts. There is need for a method of performing three-dimensional flow imaging which is not afflicted with the drawbacks associated with Doppler flow imaging.
Conventional ultrasound images are formed from a combination of fundamental and harmonic signal components, the latter of which are generated in a nonlinear medium such as tissue or a blood stream containing contrast agents. In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic signal components.
Contrast agents have been developed for medical ultrasound to aid in diagnosis of traditionally difficult-to-image vascular anatomy. The agents, which are typically microbubbles whose diameter is in the range of 1-10 micrometers, are injected into the blood stream. Since the backscatter signal of the microbubbles is much larger than that of blood cells, the microbubbles are used as markers to allow imaging of blood flow. One method to further isolate echoes from these agents is to use the (sub)harmonic components of the contrast echo, which is much larger than the harmonic components of the surrounding tissue without contrast agent. Flow imaging of (sub)harmonic signals has largely been performed by transmitting a narrowband signal at frequency .function..sub.0 and receiving at a band centered at frequency 2.function..sub.0 (second harmonic) or at frequency .function..sub.0 /2 (subharmonic) followed by conventional color flow processing. This approach has all the limitations of a conventional color flow system, namely, low resolution, low frame rate and flow sensitivity only in the axial direction.
In medical diagnostic ultrasound imaging, it is also desirable to optimize the signal-to-noise ratio (SNR). Additional SNR can be used to obtain increased penetration at a given imaging frequency or to improve resolution by facilitating ultrasonic imaging at a higher frequency. Coded excitation is a well-known radar technique used to increase signal-to-noise ratio in situations where the peak power of a transmitted signal cannot be increased but the average power can. This is often the situation in medical ultrasound imaging, where system design limitations dictate the peak amplitude of the signal driving the transducer. In this situation, longer signals can be used to deliver higher average power values, and temporal resolution is restored by correlating the return signal with a matched filter. Binary codes, or codes that can be easily represented digitally as a series of digits of +1, -1 or 0, are preferred. Binary codes are also preferred because they contain the most energy for a given peak amplitude and pulse duration.